rename ComposerHistoryManager to SendHistoryManager to avoid confusion

...with the undo history manager for the composer.

src/SendHistoryManager.js

@@ -0,0 +1,60 @@
+//@flow
+/*
+Copyright 2017 Aviral Dasgupta
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import _clamp from 'lodash/clamp';
+
+export default class SendHistoryManager {
+    history: Array<HistoryItem> = [];
+    prefix: string;
+    lastIndex: number = 0; // used for indexing the storage
+    currentIndex: number = 0; // used for indexing the loaded validated history Array
+
+    constructor(roomId: string, prefix: string = 'mx_composer_history_') {
+        this.prefix = prefix + roomId;
+
+        // TODO: Performance issues?
+        let index = 0;
+        let itemJSON;
+
+        while (itemJSON = sessionStorage.getItem(`${this.prefix}[${index}]`)) {
+            try {
+                const serializedParts = JSON.parse(itemJSON);
+                this.history.push(serializedParts);
+            } catch (e) {
+                console.warn("Throwing away unserialisable history", e);
+                break;
+            }
+            ++index;
+        }
+        this.lastIndex = this.history.length - 1;
+        // reset currentIndex to account for any unserialisable history
+        this.currentIndex = this.lastIndex + 1;
+    }
+
+    save(editorModel: Object) {
+        const serializedParts = editorModel.serializeParts();
+        this.history.push(serializedParts);
+        this.currentIndex = this.history.length;
+        this.lastIndex += 1;
+        sessionStorage.setItem(`${this.prefix}[${this.lastIndex}]`, JSON.stringify(serializedParts));
+    }
+
+    getItem(offset: number): ?HistoryItem {
+        this.currentIndex = _clamp(this.currentIndex + offset, 0, this.history.length - 1);
+        return this.history[this.currentIndex];
+    }
+}